As is known, a laser produces an intense, coherent, directional beam of light or radiation. A laser generally includes three main components: an energy ‘pump’ source, a gain medium, and a resonator cavity. The energy pump source generates a population inversion in the gain medium, and the gain medium amplifies light traveling therein. The resonator cavity typically includes a mirror disposed at each end of the gain medium, effectively defining the cavity. The pump source can be implemented, for example, with a laser diode array or flash-lamp, and the gain medium can be implemented as gas, liquid, or solid, as typically done.
During the lasing process, photons propagating along the axis of the cavity bounce back and forth across the active medium, thereby building intensity. The light in the cavity forms resonant standing waves having a frequency equal to n(c/2D), where n is the standing wave pattern or mode (e.g., 1, 2, 3, etc), c is the speed of light, and D is the distance between the mirrors. These modes are sustained in the resonator cavity, and the generated light beam is generally limited to be within the corresponding range of frequencies.
As is further known, pumping of uniformly doped gain medium tends to cause non-uniform heating or “hot-spots”, which causes internal stress/strain within the medium where the outer surfaces are cooled. This problem is exacerbated when longitudinal pumping with a single wavelength pump source. In more detail, consider the exponential decay of the transmission function: I=I0e(−αl), where I0 is the input pump beam, α is the absorption coefficient at the pump wavelength, and l is the propagation distance through the gain medium. Thus, more energy is absorbed at the beginning of the gain medium than at the end of the gain medium. Longitudinal pumping of a uniformly doped host can also cause localized thermal augmentation due to excited state absorption (ESA) and upconversion processes, both of which are exacerbated by uneven pump energy absorption.
As is further known, amplified spontaneous emission (ASE) is produced when a laser gain medium is pumped to generate a population inversion. Feedback of the ASE by the resonator cavity may produce laser operation if the lasing threshold is reached. Excessive ASE, however, is undesirable, because it limits the gain that can be achieved in the gain medium. This is particularly problematic in high gain lasers, and ASE must be mitigated in such applications so that it does not deplete the excitation of the gain medium, thereby allowing the desired coherent laser radiation to do so.
Typical solutions to such problems include using more pump diodes of varied spectral outputs, to insure some percentage of pump diodes will be in laser material absorption band, or using reduced, uniform dopant concentration and long absorption path to provide sufficient pump absorption off pump diode peak wavelength. These solutions are associated with a number of disadvantages, such as limited operating ambient temperature window, low ASE and ESA thresholds, and/or complex optical arrangements that are expensive to manufacture.
What is needed, therefore, are better laser amplifier designs.